Flow insensitive magnetization preparation pulse for T2* contrast

ABSTRACT

A magnetic resonance system comprises a magnetic resonance scanner ( 10 ) including a main magnet ( 12 ) generating a static magnetic field biasing nuclear spins toward aligning along a direction of the static magnetic field, magnetic field gradient coils ( 14 ), a radio frequency coil ( 16 ), and a controller ( 20, 22 ) configured to: (a) drive the radio frequency coil to selectively tip spins predominantly of short T2* out of the direction of the static magnetic field; (b) drive at least one of the magnetic field gradient coils and the radio frequency coil to dephase said spins predominantly of short T2* tipped out of the direction of the static magnetic field; and (c) drive the magnetic field gradient coils and the radio frequency coil to acquire magnetic resonance data that is predominantly T2* weighted due to preceding operations (a) and (b).

FIELD OF THE INVENTION

The following relates to the magnetic resonance arts, such as magneticresonance imaging, magnetic resonance spectroscopy, and so forth.

BACKGROUND OF THE INVENTION

Magnetic resonance (MR) imaging employing T2* weighting is used toprovide enhanced contrast in techniques such as blood oxygenation leveldependent (BOLD) contrast imaging, iron overload imaging, detection orimaging of superparamagnetic iron oxides (SPIO's) in molecular MRimaging, and so forth. In BOLD imaging, contrast originates from theparamagnetic nature of deoxyhaemoglobin in red blood cells, whichperturbs the main magnetic field, leading to a local reduction in mainfield homogeneity and increased T2* decay (that is, short T2*). Oxygen,on the other hand, effectively shields the paramagnetic haemoglobin,yielding an increased T2*. Thus, a contrast between oxygenated anddeoxygenated blood can be obtained by T2* weighted imaging, whichelucidates local tissue oxygen consumption.

Various species in a typical biological subject exhibit a range of T2*times. Species with long T2* times are of interest for BOLD and someother T2* weighted imaging techniques. Existing T2* weighted imagestypically rely upon the enhancement of long T2* contrast by use of longecho time (TE). For example, spoiled gradient echo sequences (SPGR) withlong echo times or echo-planar imaging (EPI) methods are used for T2*weighted imaging.

However, these sequences may not provide optimal signal-to-noise ratio(SNR), and are susceptible to flow artifacts due to the long TE. ForBOLD imaging of the brain, this may be acceptable due to typically lowflow rates in brain tissue. However, for BOLD imaging of regions offaster blood flow, such as cardiac BOLD imaging, flow artifacts can beso pronounced as to prevent successful BOLD imaging.

Accordingly, it would be useful to provide T2* weighting reflective oflong T2* species without relying upon long echo times, as is done inexisting T2* weighting approaches.

The following provides new and improved apparatuses and methods whichovercome the above-referenced problems and others.

SUMMARY OF THE INVENTION

In accordance with one disclosed aspect, a magnetic resonance method isdisclosed, comprising: dephasing spins predominantly of short T2*species without substantially dephasing spins of long T2* species;subsequent to the dephasing, performing a magnetic resonance acquisitionto acquire magnetic resonance data predominantly from long T2* species;and generating a T2* weighted image from the acquired magnetic resonancedata.

In accordance with another disclosed aspect, a storage medium storesinstructions executable to perform a magnetic resonance method as setforth in the immediately preceding paragraph. In accordance with anotherdisclosed aspect, a magnetic resonance system is configured to perform amagnetic resonance method as set forth in the immediately precedingparagraph.

In accordance with another disclosed aspect, a magnetic resonance systemis disclosed, comprising: a magnetic resonance scanner including a mainmagnet generating a static main magnetic field biasing nuclear spinstoward aligning along a direction of the static main magnetic field;magnetic field gradient coils; a radio frequency coil; and a controllerconfigured to: (a) drive the radio frequency coil to selectively tipspins predominantly of short T2* out of the direction of the mainmagnetic field; (b) drive at least one of the magnetic field gradientcoils and the radio frequency coil to dephase said spins predominantlyof short T2* tipped out of the direction of the main magnetic field; and(c) drive the magnetic field gradient coils and the radio frequency coilto acquire magnetic resonance data that is predominantly T2* weighteddue to preceding operations (a) and (b).

One advantage resides in enabling T2* weighted imaging without relianceupon imaging sequences having long echo times.

Another advantage resides in reduced flow artifacts in T2* weightedimaging.

Further advantages will be apparent to those of ordinary skill in theart upon reading and understand the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1 diagrammatically shows an illustrative magnetic resonance systemfor T2* weighted imaging.

FIG. 2 diagrammatically shows a pulse sequence for performing T2*weighted imaging.

FIG. 3 plots excitation spectra for short T2* species and for long T2*species.

FIG. 4 presents some cardiac imaging results.

Corresponding reference numerals when used in the various figuresrepresent corresponding elements in the figures.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to FIG. 1, an illustrative magnetic resonance (MR) systemincludes a magnetic resonance scanner 10 including a main magnet 12generating a static (B₀) magnetic field that is oriented along a firstdirection. In the case of a bore-type or horizontal magnet system, thefirst direction is longitudinal in orientation. For ease ofunderstanding, and by way of example, the first direction willhenceforth be referred to as the longitudinal direction, correspondingto the horizontal magnet system illustrated in FIG. 1. However, it is tobe understood that in case the main magnetic field B₀ is oriented in anyother direction, e.g., vertical, then the first direction would be takento mean the direction of the main magnetic field B₀. In the illustratedMR system, the static or main magnetic field B₀ biases nuclear spinstoward aligning along the longitudinal direction. The illustrated mainmagnet 12 is a superconducting magnet kept at superconductingtemperature by immersion in liquid helium LH. The MR scanner 10 furtherincludes magnetic field gradient coils 14 for selectively superimposingmagnetic field gradients onto the static B₀ magnetic field, for use inspatially encoding or otherwise manipulating magnetic resonance. A radiofrequency (RF) coil 16 is energized during an excitation phase to excitemagnetic resonance and detects the excited magnetic resonance during areadout phase of a magnetic resonance pulse sequence. In the illustratedembodiment, the single illustrated RF coil 16 is a birdcage coil;however, other types of coils or coil arrays can be used fortransmission or receiving. In some embodiments, separate transmit andreceive coils or coil arrays are provided. The illustrative MR scanner10 is a horizontal-bore cylindrical-type scanner; however, the disclosedT2* weighted imaging techniques can be practiced with open MR scanners,vertical MR scanners, or substantially any other type of MR scanner.

The MR scanner 10 is controlled by an MR controller 20 in accordancewith a pulse sequence, such as an illustrated T2* weighted pulsesequence 22, to acquire T2* weighted magnetic resonance data that arestored in an MR data memory 24. If the acquired T2* weighted magneticresonance data are suitably spatially encoded, then an MR datareconstruction processor 26 optionally processes the acquired T2*weighted magnetic resonance data to generate a T2* weighted image fromthe acquired magnetic resonance. For example, the acquired T2* weightedmagnetic resonance data may be spatially encoded using slice-selectivegradients applied during the magnetic resonance excitation phase,readout gradients applied during the readout phase, and phase encodinggradient applied between the excitation and readout phases in order toacquire T2* weighted magnetic resonance data in the form of k-space datathat are suitably reconstructed by the reconstruction processor 26 usinga Fourier Transform based reconstruction algorithm. The generated T2*weighted MR images are suitably stored in an MR images memory 28,displayed on a display 30, or otherwise utilized.

The processing and memory components 20, 22, 24, 26, 28 are suitablyembodied as a computer 32 having said display 30, as in the illustratedembodiment, or as another digital processing device such as a networkserver or so forth. The MR controller 20 configured to implement the T2*weighted pulse sequence 22 is suitably implemented by the computer 32 oranother digital processor, or may be embodied as a storage medium suchas a magnetic disk, optical disk, random-access memory (RAM), read-onlymemory (ROM), or so forth storing instructions executable by thecomputer 32 or another digital processor to cause the MR scanner 10 toperform the T2* weighted pulse sequence 22 to acquire magnetic resonancedata. Moreover, although an imaging application is described, thedisclosed T2* preparation sequences can be employed for other MR dataacquisitions, such as MR spectroscopy.

With reference to FIG. 2, an illustrative embodiment of the T2* weightedpulse sequence 22 is described. Some existing T2* weighted pulsesequences operate based on the use of a long echo time, with theobjective of allowing T1, T2, and short T2* weighted spins tosubstantially decay so that the readout signal is strongly T2* weightedbased on species having long T2* times. The T2* weighted pulse sequence22 operates in a fundamentally different fashion, namely by employing amagnetization preparation pulse sub-sequence 40 that dephases spinspredominantly of short T2* species without substantially dephasing spinsof long T2* species, and following this with substantially any type ofMR imaging sequence 42. Since the short T2* species are dephased, the MRimaging sequence 42, regardless of its type, generates a T2* weightedimage that is weighed towards long T2* contrast. The magnetizationpreparation pulse sub-sequence is optionally spatially nonselective, asin the illustrated embodiment of FIG. 2, and the imaging sequence 42optionally employs a short echo time (that is, short TE), in which casethe resulting T2* weighted image is substantially free of flowartifacts.

With continuing reference to FIG. 2 and with further reference to FIG.3, operation of the magnetization preparation pulse sub-sequence 40 thatdephases spins predominantly of short T2* species without substantiallydephasing spins of long T2* species is based on the observation thatshort T2* species have substantially broader excitation spectra than dolong T2* species, as shown in FIG. 3. (The axis marked F denotes thefrequency axis.) As a result, an excitation pulse having a widebandwidth □F₁ equal to or larger than the bandwidth of the short T2*species excitation spectrum will manipulate all nuclear species,regardless of whether they have short T2* decay time or long T2* decaytime. On the other hand, an excitation pulse having a narrow bandwidth□F₂ comparable with or smaller than the bandwidth of the long T2*species spectrum will manipulate only the long T2* species. Based onthis observation, a combination of narrow bandwidth and wide bandwidthRF pulses can be combined to excite the short T2* nuclear specieswithout exciting the long T2* nuclear species. A magnetic field gradientcan then be applied to dephase the excited T2* spins without affectingthe long T2* spins. Thereafter, substantially any magnetic resonanceimaging pulse sequence can be applied, and it will result in imagingpredominated by the long T2* spins but not the dephased short T2* spins.

With particular reference to FIG. 2, the magnetization preparationsequence 40 includes a narrow bandwidth excitation pulse 50. In thepresent example, an excitation pulse with a tip angle of 90° isselected; other tip angles greater or smaller than 90° are equallypossible. For example, in some embodiments the tip angle is in the range60°-120°, although tip angles outside this range are also contemplated.To achieve a narrow bandwidth, the excitation pulse 50 is suitably oflong temporal duration, for example having a duration that is about orlarger than the T2* of the short T2* species. The effect of the narrowbandwidth 90° excitation pulse 50 is to tip spins predominantly of longT2* species out of a longitudinal direction, for example to 90° tipangle in the case of the illustrated 90° excitation pulse 50. On theother hand, short T2* species are substantially unaffected by the narrowbandwidth 90° excitation pulse 50. As seen in FIG. 3, there will be asmall fraction of short T2* species whose excitation response does fallwithin the narrow bandwidth of the narrow bandwidth 90° excitation pulse50; however, the excited spins are predominantly of the long T2*species.

The narrow bandwidth 90° excitation pulse 50 is followed by a widebandwidth spin refocusing pulse 52. In the present example, a refocusingpulse with a tip angle of 180° is selected; other tip angles greater orsmaller than 180° are equally possible. For example, in some embodimentsthe tip angle is in the range 150°-210°, although tip angles outsidethis range are also contemplated. To achieve a wide bandwidth, therefocusing pulse 52 is suitably of short temporal duration, for examplehaving has a duration that is shorter than T2* of the short T2* species.The effect of the wide bandwidth 180° spin refocusing pulse 52 is torefocus the spins predominantly of long T2* species that were tipped outof the longitudinal direction by the narrow bandwidth 90° excitationpulse 50. At the same time, short T2* species are also manipulated bythe wide bandwidth 180° refocusing pulse 52. The effect is to tip theshort T2* species 180° out of the longitudinal direction and into aninverted direction (that is, antiparallel with the longitudinaldirection).

If the wide bandwidth 180° refocusing pulse 52 is centered a timeinterval Δt after the center of the narrow bandwidth 90° excitationpulse 50, then the refocusing effect is centered at a time Δt after thecenter of the wide bandwidth 180° refocusing pulse 52. In other words,the spins predominantly of long T2* species that were tipped out of thelongitudinal direction by the narrow bandwidth 90° excitation pulse 50are most strongly refocused at time Δt after the center of the widebandwidth 180° refocusing pulse 52.

At the refocusing time (that is, at time Δt after the center of the widebandwidth 180° refocusing pulse 52), a wide bandwidth 90° restoringpulse 54 returns the refocused spins predominantly of long T2* speciesback to the longitudinal direction. Just as the previously describedexcitation pulse 50, a restoring pulse having a different tip anglesthan 90° is equally possible. For example, in some embodiments the tipangle is in the range 60°-120°, although tip angles outside this rangeare also contemplated. Again, the wide bandwidth restoring pulse 54 issuitably of short temporal duration, for example having a duration thatis shorter than T2* of the short T2* species, so as to have asufficiently wide bandwidth to affect both long and short T2* species.Accordingly, the wide bandwidth restoring pulse 54 also manipulates theshort T2* species. These short T2* species were inverted (tip angle˜180°) by the wide bandwidth 180° refocusing pulse 52. The 90° restoringpulse 54 therefore imparts a 90° tip angle to the short T2* species.

In summary, the combined effect of the narrow bandwidth 90° excitationpulse 50, the wide bandwidth 180° refocusing pulse 52, and the widebandwidth restoring pulse 54, is that the long T2* species arepredominantly in the longitudinal direction while the short T2* speciesare predominantly at 90° tip angle, that is, tipped into the transversalplane that is transverse to the longitudinal direction. The pulses 50,52, 54 are then followed by a crusher magnetic field gradient 56 thatdephases or crushes the excited spins, that is, the short T2* speciesthat are predominantly at 90° tip angle. On the other hand, the crushermagnetic field gradient 56 has substantially no effect on the long T2*species that are longitudinally oriented and hence are not excitedspins.

To summarize, the T2* contrast preparation pulse sub-sequence 40includes the long-duration 90° excitation pulse 50 with narrowbandwidth, which tips the magnetization of species with long T2*(narrowspectrum, see FIG. 3) into the transversal plane. Next, the short 180°refocusing pulse 52 with wide bandwidth refocuses magnetization of thesepredominantly long T2* species, and inverts the other (predominantlyshort T2*) species. The 90° restoring RF pulse 54 with wide bandwidthrestores the refocused magnetization of species predominantly with longT2* to the longitudinal direction, while the inverted magnetization ofspecies with predominantly short T2* is tipped into the transversalplane and spoiled by the gradient spoiler 56. The illustrative pulsesequence 50, 52, 54, 56 ensures that predominantly species with long T2*are available for imaging, while species with short T2* are effectivelysuppressed. As a result, the succeeding imaging pulse sub-sequence 42yields a strong T2* contrast. This is true even if the succeedingimaging pulse sub-sequence 42 does not have a long echo time (that is,even if the imaging pulse sub-sequence 42 has short TE). The strong T2*contrast is achieved not by using a long TE, but rather by effectivelysuppressing species with short T2*. (However, it is also contemplatedfor the imaging pulse sub-sequence 42 to have a long TE, in which casestrong T2* contrast is provided both by the preparation sub-sequence 40and by the long TE).

Advantageously, the magnetization preparation sub-sequence 40 is notspatially selective, that is, no slice selection gradients are employed.As a result, the short T2* species are suppressed in the wholeexcitation volume, rather than only in a selected slice, whichsubstantially suppresses or eliminates flow artifacts even in regions offast blood flow such as the cardiac region. Because the preparationsub-sequence 40 provides the T2* contrast, the following imagingsequence 42 used to collect magnetic resonance imaging data can be ofsubstantially any type. For example, the MR imaging sub-sequence 42 canbe a selected fast MR sequence with optimal SNR and low susceptibilityto flow artifacts as may be appropriate for a selected application.

In one example, the imaging sub-sequence 42 may have a short echo timeeffective to provide predominantly proton density weighted imaging inthe absence of the short T2* dephasing provided by the preparatorysequence 40. When used in conjunction with the preparatory sequence 40,that same imaging sequence provides a T2* weighted image, rather than aproton density weighted image. This is a consequence of the suppressionof short T2* species by the magnetization preparation sub-sequence 40.

The narrow bandwidth or wide bandwidth of the RF pulses 50, 52, 54 canbe achieved by adjusting pulse duration. Typically, it is desired for aratio of the wide bandwidth to the narrow bandwidth to be at least abouttwo, which reflects the typical ratio between the wide bandwidth of theshort T2* spectrum and the narrow bandwidth of the long T2* spectrumdiagrammatically shown in FIG. 3.

The sequence of RF pulses 50, 52, 54 shown in FIG. 2 is advantageous inthat it uses spin refocusing to keep the excited spins of predominantlylong T2* species well-focused at the time the restoration RF pulse 54 isapplied. However, other magnetization preparation sequences are alsocontemplated for producing the desired final result of preferentialexcitation of short T2* species over long T2* species. For example,another contemplated preparation sequence is to use the illustratednarrow bandwidth 90° excitation RF pulse 50 to tip predominantly thelong T2* species into the transverse plane, and then to follow directlywith a wide bandwidth 270° RF pulse which tips the spins ofpredominantly long T2* species lying in the transverse plane back intothe longitudinal direction, while simultaneously tipping the short T2*species from the longitudinal direction into the transverse plane. Thissequence can again be followed by the illustrated crusher gradient 56 tospoil or dephase excited spins of predominantly short T2* species.Additionally or alternatively, radio frequency (RF) spoiling applied bythe RF coil 16 can be substituted for the spoiling gradient 56 appliedby the gradient coils 14.

With reference to FIG. 4, the combination of the spatially non-selectivemagnetization preparation sub-sequence 40 with the subsequent imagingsub-sequence 42 having a short TE provides good temporal resolution, andcan enable T2* weighted imaging contrast techniques to be employed insituations where heretofore these techniques have not been practical.For example, BOLD imaging of the heart has heretofore been substantiallylimited by flow artifacts caused by rapid blood flow in the cardiacregion. FIG. 4 shows an in vivo example of imaging of a human heart(short axis view). In the leftmost image labeled “[a]” in FIG. 4, agradient echo sequence was used with a short echo time (TE), and a smallflip angle was used for imaging. This provides predominantly protondensity weighting. As a result, no contrast between the deoxygenatedblood in the right ventricle (labeled “RV” in FIG. 4) and the oxygenatedblood in the left ventricle (labeled “LV” in FIG. 4) is observed in theimage labeled “[a]”.

In the middle image of FIG. 4 labeled “[b]”, the echo time was increasedto TE=15 ms with the aim to provide T2* weighting. However, signal voidsin the ventricles and flow artifacts along the phase encoding directionare present as a result of the long TE, yielding undiagnostic imagequality.

In the rightmost image of FIG. 4 labeled “[c]”, the same sequence asused for the leftmost image “[a]” with short TE was employed, but inthis case the image acquisition was preceded by the T2* preparationpulse sequence 40 of FIG. 2. The deoxygenated blood in the rightventricle (which has short T2*) appears almost completely suppressed,while the signal from the oxygenated blood in the left ventricle (whichhas long T2*) is not attenuated. No substantial flow artifacts areobserved.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof. In the claims, anyreference signs placed between parentheses shall not be construed aslimiting the claim. The word “comprising” does not exclude the presenceof elements or steps other than those listed in a claim. The word “a” or“an” preceding an element does not exclude the presence of a pluralityof such elements. The disclosed embodiments can be implemented by meansof hardware comprising several distinct elements, or by means of acombination of hardware and software. In the system claims enumeratingseveral means, several of these means can be embodied by one and thesame item of computer readable software or hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

Having thus described the preferred embodiments, the invention is nowclaimed to be:
 1. A magnetic resonance imaging method, comprising:controlling a magnetic resonance imaging system to generate amagnetization preparation pulse subsequence, wherein the magnetizationpreparation pulse subsequence includes: a narrow bandwidth excitationpulse having a duration which is longer than a dephasing time T2* of ashort T2* species and which tips spins predominantly of long T2* speciesout of a longitudinal direction of a main magnetic field whilepredominantly leaving the short T2* species unaffected, a wide bandwidthspin refocusing pulse having a duration which is shorter than thedephasing time T2* of the short T2* species and which predominantlyrefocuses the long T2* species and predominantly inverts spins of theshort T2* species, and a wide bandwidth restoring pulse having aduration which is shorter than the dephasing time T2* of the short T2*species and which predominantly returns spins of the long T2* speciesback to the longitudinal direction of the main magnetic field andpredominantly tips the spins of the short T2* species out of thelongitudinal direction of the main magnetic field, wherein the narrowbandwidth excitation pulse, the wide bandwidth spin refocusing pulse,and the wide-bandwidth restoring pulse all have a same center frequencyas each other; controlling the magnetic resonance imaging system togenerate an imaging pulse subsequence to perform magnetic resonanceimaging data acquisition following the magnetization preparation pulsesubsequence; and processing, by the magnetic resonance imaging system,the magnetic resonance imaging data, wherein the magnetic resonanceimaging data provides blood oxygenation level dependent contrast.
 2. Themagnetic resonance imaging method of claim 1, wherein the magneticresonance imaging data acquisition employs a short echo time effectiveto provide predominantly proton density weighted imaging in the absenceof the dephasing.
 3. The magnetic resonance imaging method of claim 1,wherein the magnetic resonance imaging data acquisition employs a shortecho time that is short compared with T2* of the long T2* species. 4.The magnetic resonance imaging method of claim 1, wherein a ratio of (i)a bandwidth of the wide bandwidth spin refocusing pulse and (ii) abandwidth of the narrow bandwidth excitation pulse is at least abouttwo.
 5. The magnetic resonance imaging method of claim 1, wherein thenarrow bandwidth excitation pulse has a 90° flip angle, the widebandwidth spin refocusing pulse has a 180° flip angle, and the widebandwidth restoring pulse has a 90° flip angle.
 6. The magneticresonance method of claim 1, wherein the narrow bandwidth excitationpulse has a flip angle of between 60° and 120° , the wide bandwidth spinrefocusing pulse has a flip angle of between 150° and 210° , and thewide bandwidth restoring pulse has a flip angle between 60° and 120°. 7.The magnetic resonance imaging method of claim 1, wherein dephasing ofthe short T2* species is spatially nonselective.
 8. The method of claim1, wherein the magnetization preparation pulse subsequence furtherincludes a spoiler pulse after the narrow bandwidth excitation pulse,the wide bandwidth spin refocusing pulse, and the wide bandwidthrestoring pulse, wherein the spoiler pulse dephases the short T2*species spins tipped out of the longitudinal direction of the mainmagnetic field.
 9. The method of claim 8, wherein the spoiler pulse is acrusher gradient pulse.
 10. The method of claim 8, wherein the spoilerpulse is an RF pulse.
 11. The method of claim 1, wherein a time intervalΔt between a center of the narrow bandwidth excitation pulse and acenter of the wide bandwidth refocusing pulse equals a time interval Δtbetween center of the wide bandwidth refocusing pulse and the center ofthe wide bandwidth restoring pulse.
 12. A magnetic resonance imagingsystem, comprising: a magnetic resonance scanner including a main magnetgenerating a static, main magnetic field biasing nuclear spins towardaligning along a direction of the static main magnetic field; magneticfield gradient coils; a radio frequency coil; and a controllerconfigured to: drive the radio frequency coil to generate amagnetization preparation pulse subsequence including: a narrowbandwidth excitation pulse having a duration which is longer than adephasing time T2* of a short T2* species and which tips spinspredominantly of long T2* species out of a longitudinal direction of amain magnetic field while predominantly leaving the short T2* speciesunaffected, a wide bandwidth spin refocusing pulse having a durationwhich is shorter than the dephasing time T2* of the short T2* speciesand which predominantly refocuses the long T2* species and predominantlyinverts spins of the short T2* species, and a wide bandwidth restoringpulse having a duration which is shorter than the dephasing time T2* ofthe short T2* species and which predominantly returns spins of the longT2* species back to the longitudinal direction of the main magneticfield and predominantly tips the spins of the short T2* species out ofthe longitudinal direction of the main magnetic field, wherein thenarrow bandwidth excitation pulse, the wide bandwidth spin refocusingpulse, and the wide-bandwidth restoring pulse all have a same centerfrequency as each other, and drive the magnetic field gradient coils andthe radio frequency coil to acquire magnetic resonance data that ispredominantly T2* weighted in response to the magnetization preparationpulse subsequence, wherein acquiring the magnetic resonance data that ispredominantly T2* weighted provides blood oxygenation level dependentcontrast imaging.
 13. The magnetic resonance imaging system of claim 12,wherein the controller is further configured to drive the magnetic fieldgradient coils to apply a spoiler pulse after the narrow bandwidthexcitation pulse, the wide-bandwidth spin refocusing pulse, and the widebandwidth restoring pulse, wherein the spoiler pulse dephases the shortT2* species spins tipped out of the longitudinal direction of the mainmagnetic field.
 14. The magnetic resonance imaging system of claim 13,wherein the spoiler pulse is a crusher gradient pulse.
 15. The magneticresonance imaging system of claim 13, wherein the spoiler pulse is aradio frequency pulse.
 16. The magnetic resonance imaging system ofclaim 12, wherein a time interval Δt between a center of the narrowbandwidth excitation pulse and a center of the wide bandwidth refocusingpulse equals a time interval At between center of the wide bandwidthrefocusing pulse and the center of the wide bandwidth restoring pulse.17. The magnetic resonance imaging system of claim 12, wherein thenarrow bandwidth excitation pulse has a 90° flip angle, the widebandwidth spin refocusing pulse has a 180° flip angle, and the widebandwidth restoring pulse has a 90° flip angle.
 18. The magneticresonance imaging system of claim 12, wherein the narrow bandwidthexcitation pulse has a flip angle of between 60° and 120°, the widebandwidth spin refocusing pulse has a flip angle of between 150° and210°, and the wide bandwidth restoring pulse has a flip angle between60° and 120°.
 19. A magnetic resonance imaging method, comprising:controlling a magnetic resonance imaging system to generate amagnetization preparation pulse subsequence, wherein the magnetizationpreparation pulse subsequence includes: a narrow bandwidth excitationpulse having a duration which is longer than a dephasing time T2* of ashort T2* species and which tips spins predominantly of long T2* speciesout of a longitudinal direction of a main magnetic field whilepredominantly leaving the short T2* species unaffected, a wide bandwidthspin refocusing pulse having a duration which is shorter than thedephasing time T2* of the short T2* species and which predominantlyrefocuses the long T2* species and predominantly inverts spins of theshort T2* species, and a wide bandwidth restoring pulse having aduration which is shorter than the dephasing time T2* of the short T2*species and which predominantly returns spins of the long T2* speciesback to the longitudinal direction of the main magnetic field andpredominantly tips the spins of the short T2* species out of thelongitudinal direction of the main magnetic field; generating, using themagnetic resonance imaging system, an imaging pulse subsequencefollowing the magnetization preparation pulse subsequence; acquiring,using the magnetic resonance imaging system, magnetic resonance imagingdata that is predominantly T2* weighted from a human heart produced inresponse to the magnetization preparation pulse subsequence and theimaging pulse subsequence; and processing the acquired magneticresonance imaging data to provide blood oxygenation level dependentcontrast imaging of the human heart.